1. Field of the Invention
The present invention relates to electric rotary machines or alternators to be mounted to motor vehicles such as passenger vehicles and trucks.
2. Description of the Related Art
An alternator mounted to a motor vehicle generates electric power by a driving force transmitted from an internal combustion engine of the motor vehicle. Output terminals of the alternator are connected to terminals of a battery mounted to the motor vehicle through charging wires. The alternator supplies the generated electric power to the battery through the charging wires. The alternator further supplies the generated electric power to various electrical loads mounted to the motor vehicle through other wires. When the charging wires are disconnected from the output terminals of the alternator or the terminals of the battery while the alternator is running, i.e. during the electric power generation, a highly transient voltage is generated. Such a transient voltage is called as a load dump. When the disconnection of the charging wires occurs, the generated highly transit voltage as the load dump reaches 100 volts, for example. As a result, the electrical loads mounted to the motor vehicle and various components in the alternator are often damaged by generated load dump surges. In order to protect the electrical loads and various components in the alternator from such a highly transient voltage, it is necessary to have a countermeasure against the load dump surges.
In order to solve such a problem, conventional techniques have disclosed various types of load dump protection devices. For example, Japanese patent laid open publication number JP H09-219938 discloses an alternator for a motor vehicle in which a lower arm of a bridge circuit in a rectifying section for the alternator is composed of MOS transistors as switching elements. The MOS transistors are turned on simultaneously when a load dump occurs and an output voltage of the alternator exceeds a reference voltage. This makes it possible to execute a load dump protection operation which suppresses the generation of a highly output voltage in the alternator when a load dump occurs. However, in the structure of the conventional alternator disclosed in JP H09-219938, when the output voltage of the alternator decreases to not more than the reference voltage after the MOS transistors as the switching elements in the lower arm of the bridge circuit are turned on, each of the MOS transistors in the lower arm of the bridge circuit is turned off again in order to re-execute a usual rectifying operation.
By the way, in the structure of the conventional alternator disclosed in JP H09-219938 as previously described, there is a delay time counted from a time when an actual output voltage of the alternator exceeds the reference voltage to a time when the load dump protection device executes the load dump protection operation. In addition, it is also necessary to have a low pass filter in order to eliminate noise and prevent the load dump protection device from executing incorrect operation caused by noise. Still further, it is necessary to consider a response time of an internal circuit components in the alternator in addition to the above delay time. During the delay time until the MOS transistors are turned on, the energy stored in the stator of the alternator is outputted to the output terminals of the alternator, and this outputted energy suddenly increases a terminal voltage of a capacitance which is used for eliminating noise. As a result, the MOS transistors in the bridge circuit and the internal circuit are damaged by the increased terminal voltage of the alternator. This decreases the reliability of the alternator. After the MOS transistors in the bridge circuit is turned on during the load dump protection operation, because the outputted energy supplied from the stator is prohibited and the control circuit of the MOS transistors and a control circuit of the alternator consume the output voltage of the alternator, the load dump protection circuit stops the execution of the load dump protection operation when the output voltage of the alternator decreases to not more than a predetermined voltage. The execution of the load dump protection operation and the stop of the load dump protection operation are repeated until the energy stored in the stator is consumed. However, when the capacitor connected to the output terminal of the alternator has a small capacitance, the number of repetitions to repeatedly execute the load dump protection operation and stop the load dump protection operation increases, a power loss is accordingly accumulated. As a result, there is a possibility of damaging the MOS transistors in the bridge circuit. In particular, such occurrence of damage extremely increases when the charging wires are disconnected from the output terminals of the alternator. Accordingly, there is a strong demand for the alternator to have a countermeasure device capable of protecting the MOS transistors as switching elements in the bridge circuit from being damaged when a load dump occurs.